Room Paint Coverage Calculator

The basic math

wall area = 2 × (length + width) × height − doors and windows coverage per gallon ≈ 350 sq ft (one coat, smooth surface) gallons needed = (wall area × number of coats) ÷ 350

Always round up. A 12×15 foot bedroom with 9 ft ceilings, one door, two windows, painted in two coats:

• Wall area: 2 × (12+15) × 9 = 486 sq ft
• Minus 1 × 20 (door) + 2 × 15 (windows) = 436 sq ft net
• Two coats: 872 sq ft of coverage needed
• Gallons: 872 ÷ 350 = 2.49
• Add 10% waste: 3 gallons (always round up)

The 350 sq ft rule is optimistic

Paint cans say “covers 400 sq ft” because that’s the lab number. In reality, with cutting in, roller absorption, drying-too-fast in summer, or stippling over textured drywall, you get closer to 300 to 350 sq ft per gallon. Old plaster, raw drywall, and dark-to-light color changes drink paint at 250 sq ft/gallon. Plan conservatively.

When you need more than two coats

• Dark color over white: usually fine in two coats, but reds and yellows often need three (they’re translucent pigments)
• White over dark: almost always three coats. Use a tinted primer first to cut a coat off
• New drywall: PVA primer + 2 coats of paint
• Glossy surface being painted with flat: prime first or paint will streak
• Smoke or water damage: shellac-based primer (Zinsser BIN), then 2 coats

Real coverage by paint type

Door and window deductions — keep them simple

The standard adjustments most contractors use:

• Door: 20 sq ft (covers a 3’ × 6'8" door + trim allowance)
• Window: 15 sq ft (standard 3’ × 5')
• Sliding glass door: 40 sq ft
• Closet door (bi-fold): 12 sq ft
• Built-in bookcase: roughly its face area

Some painters skip deductions entirely on small openings — they figure the leftover paint covers touch-ups anyway. Fine for small jobs; not fine for a large wall with multiple windows.

Why buying the right amount matters more than you think

Paint stores can usually re-mix a custom color, but never identically. Tint formulas drift slightly between batches even from the same store. If you run out mid-job and have to buy another gallon a week later, the new paint can show as a faintly different shade on the wall — especially in raking sunlight. Always buy a full gallon extra over the calculator’s number for one-coat surfaces, half a gallon extra on two-coat jobs. Keep the leftover for touch-ups; the can lasts 2 to 5 years sealed properly.

Sample sizes — the smartest $4 you’ll spend

Most paint stores sell 8 oz sample pots. Paint a 2×2 ft swatch on the wall, then look at it morning, noon, and night before committing to 3 gallons of a color. Wall paint looks very different from the chip in store lighting. Most professional painters do this routinely; most homeowners skip it and regret it.

Pricing rough guide (2024 US)

A 3-gallon bedroom job at mid-tier paint costs $105 to $165 plus brushes, rollers, drop cloths, painter’s tape, and primer if needed. Budget another $50 to $100 in supplies if you don’t have them already.

The bigger time sink than you expect

Most DIY painters underestimate prep time by 50 to 100%. Filling holes, sanding, taping, cleaning, removing outlet covers — that’s typically half the total job. Actual rolling and cutting in is faster than the prep. Block out a full weekend for a single room, not a Saturday afternoon.